Field of the Invention
The invention relates to an overvoltage protection device with a housing, with two terminals for electrical connection of the overvoltage protection device to the current path which is to be protected, with at least one arrester, in particular a varistor, which is located within the housing, in the normal state of the overvoltage protection device the first terminal being connected in an electrically conductive manner to the first connecting region of the arrester and the second terminal being connected to the second connecting region of the arrester via at least one component.
Description of Related Art
Electrical circuits and systems normally work without perturbations with the voltage specified for them, the rated voltage. This does not apply when overvoltages occur. Overvoltages are all voltages which are above the upper tolerance limit of the rated voltage. They include mainly also the transient overvoltages which can occur due to atmospheric discharges, but also due to switching operations or short circuits in power supply networks and can be coupled conductively, inductively or capacitively into electrical circuits. In order to protect electrical or electronic circuits wherever they are also used, against transient overvoltages, overvoltage protection devices were developed and have been used for decades.
Due to ageing and temporary overvoltages TOV in the range of seconds, an unwanted increase of the leakage current of the varistor at operating voltages occurs especially in overvoltage protection devices with a varistor as the arrester. Overvoltage protection devices with a varistor as the arrester therefore nowadays often have a thermal disconnector by which a varistor which is no longer properly operable is electrically disconnected from the current path to be monitored. In the known overvoltage protection devices, the state of the varistor is monitored according to the principle of a temperature switch, when the varistor overheats, for example, due to leakage currents which have occurred, a solder connection provided between the varistor and a disconnection means is broken; this leads to electrical disconnection of the varistor.
This overvoltage protection device is known, for example, from German Patent Application DE 695 03 743 T2. In the known overvoltage protection device which has two varistors which are located parallel to one another, the thermal disconnector is additionally connected to an optical status display so that the state of the overvoltage protection element can be read off directly on site using the optical status display. As the optical status display, this overvoltage protection device has a first slide which is located in the housing and which is actuated by separating tongues which form the isolating means, and interacts with a second slide which can be moved relative to a viewing port depending on the position of the first slide.
German Patent Application DE 601 12 410 T2 and corresponding U.S. Pat. No. 6,430,020 B1 disclose an overvoltage protection device which has a varistor disk wafer which is located in a metal housing and which is braced against the bottom of the pot-shaped housing a piston-shaped electrode. The housing is closed with a cover which is either screwed into the pot-shaped housing or is attached by a spring ring or a clip which latches in a groove in the side wall of the housing. There is an opening in the cover through which the shaft of the electrode is routed out of the housing for electrical connection of the electrode. The second terminal for electrical connection of the overvoltage protection device to the current paths or signal paths which are to be protected is made on the housing. For electrical insulation of the electrode relative to the housing, there is an insulating ring which is located within the housing and which likewise has an opening for the shaft of the electrode.
German Patent Application DE 10 2007 030 653 A1 and corresponding U.S. Pat. No. 8,089,745 B2 disclose an overvoltage protection device which, likewise, has a metal housing consisting of two housing half shells, in the housing there being preferably two varistors which are connected in parallel to one another and one central electrode which is located between the varistors. The two varistors can be forced away from the central electrode by a thermal disconnector which is located in the housing, as a result of which the varistors are disconnected from the current path.
The above described thermal disconnectors which are often used in the known overvoltage protection devices and which are based on the melting of a solder connection have the disadvantage that they have a relatively inert response behavior; this can lead to the fact that rapidly rising fault currents, which due to damage to the overvoltage protection device flow via the latter, do not lead promptly enough to breaking the solder connection. If the impedance of a faulty arrester has changed so dramatically that high line-driven fault currents flow via the arrester, the high power transfer resulting therefrom within the overvoltage protection device which has become low-resistance heat the latter so quickly and so dramatically that open arcs can occur before the thermal disconnector has disconnected the overvoltage protection device.
To prevent these fault cases, in practice, separate back-up fuses, for example, fusible links, are used as short-circuit current protection; they must be matched to the discharge capacity of the overvoltage protection device. But, it is disadvantageous here that, on the one hand, an additional component is needed with the back-up fuse, that on the other hand these promptly reacting back-up fuses have only a limited impulse current-carrying capacity.
German Patent Application DE 10 2010 015 814 A1 and corresponding U.S. Pat. No. 8,766,762 B2 disclose an overvoltage protection device of the type initially described in which, in a two-part housing, there are two varistors which are connected in parallel to one another and a gas-filled surge arrester which is connected in series to the varistor as a component. In addition, in one preferred version of the known overvoltage protection device, a respective fusible link is assigned to the two varistors as a back-up fuse. By integrating the fusible link into the housing of the overvoltage protection device, the use of additional separate back-up fuses can be eliminated. However, the disadvantage here is also that the fusible links are only conditionally power pulse current-proof according to the characteristic fusing integral.
As was already stated at the beginning, due to ageing and frequent pulsed loading, in varistors at the end of their lives the insulating properties are diminished, as a result of which a power loss is converted in the varistor; this leads to heating of the varistor. The temperature of a varistor can rise so dramatically that there is a risk of fire. In order to prevent such dramatic heating, in the known overvoltage protection device, in addition there is still at least one temperature-dependent, short-circuit switch, the short-circuit switch or switches however short circuiting the varistor or varistors. Altogether, the known overvoltage protection device thus has a host of components, to achieve the described advantages in particular two varistors which are connected in parallel being necessary.